Radiation counting tube of the geiger-muller type

ABSTRACT

A radiation counting tube includes a plurality of longitudinally disposed cathode sections of different sensitivities coaxially surrounding a common anode to permit simplified measurement of a wide range of radiation on a single meter scale without switching.

United States Patent Maillot [451 May 23, 1972 54 RADIATION COUNTINGTUBE OF THE 2,917,647 12/1959 Fowler 61 al ..250/83.6 R x GEIGER-MULLERTYPE 3,259,775 7/1966 Nienhuis et al ..250/83.6 R x 3,346,754 lO/1967Natanagara et al. .....250/83.6 R X [72] invent France 3,478,205 11/1969Sporek ..250/83.6 R x [73] Assignee: International Standard ElectricCorporation, New York, NY. Primary ExaminerArchie R. BorcheltAttorney-C. Cornell Remsen, Jr. Walter J. Baum Paul W.

22 197 [22] Filed Jan 0 Hemminger, Percy P. Lantzy, Philip M. Bolton,Isidore Togut [2!] Appl. No.: 4,968 and Charles L. Johnson, Jr.

52 us. 01. ..250/83.6 R, 313/93 [571 ABSTRACT [51] IIIIL Cl. ..G0lt l/l8A radiation counting tube includes a plurality of longitudinally [58]Fleld of Search ..250/83.6 R; 313/93 disposed cathode sections ofdifferent sensitivities coaxially References Cited surrounding a commonanode to penmt s1rnpi1fied measurement of a wide range of radlauon on as1ngle meter scale UNITED STATES PATENTS swltchmg- 2,657,3 l 5 10/ l 953Goldstein ..250/83.6 R 10 Clains, 5 Drawing Figures Patented May 23,1972 5 Sheets-Sheet l lnvenlor JEAN-PAUL MA/LLOT ltwml Home y PatentedMa 23, 1972 5 Sheets-Sheet 2 oow I nvenlor JEAN-PAUL MAILLOT PatentedMay 23, 1972 5 Sheets-Sheet :5

, lnvenlor JEAIYPAUL MA/L LOT By v Attorney Patented May 23, 19723,665,189

5 Sheets-Sheet 4 Inventor JEAN-PA UL MAIL LOT lwwl A Uorney Patented May23, 1972 3,665,189

5 Sheets-Sheet 5 I nvenlor JEAN'PA UL MAIL-LOT A Home y RADIATIONCOUNTING TUBE OF THE GEIGER- MULLER TYPE BACKGROUND OF THE INVENTION 1.Field of the Invention The present invention relates to radiationcounting tubes of the Geiger-Muller type and particularly to a tubehaving a plurality of sensitive elements with different sensitivities topermit measurements over a wide range of nuclear radiation.

Counting tubes of this type are primarily used in radiometers designedfor simple measurement of amounts of gammaradiation over a wide range ofvalues, between 0.001 and i000 Roentgens per hour. Gamma radiation at aparticular location is measured in Roentgens, R. A Roentgen is definedas an amount of -y-radiation such that the corpuscular emission which isassociated therewith generates in air, per cubic centimeter of air atnormal temperature and pressure, ions which convey a quantity ofelectricity of either sign equal to the electrostatic metric unit or0.33-' Coulombs. The delivery of radiation amounts, existing at a givenlocation, is generally measured in Roentgens per hour (R/h).

2. Description of the Prior Art Several types of radiometers arepresently known that allow measurement of the delivery of 'y-radiationamounts. The desired qualities for these devices are:

a. good precision (from i 10% to i 20%),

b. good stability over a period of time,

c. good resistance to environmental conditions,

d. self-sufficiency and small weight, so as to make the apparatusportable, and which implies very simple electronic circuits,

e. low cost, especially in military applications where it is desirableto reach to a widespread area,

1'. reading simplicity which will not require any interpretation, andminimum range switching with a large dynamic range,

. reliable operation without errors which may be caused either bysaturation of the 'y-radiation detector or by failure of the detector orany other apparatus components, and also simple electronic circuitry.

ln the presently existing radiometers, the above mentioned qualities donot occur together. For instance, some radiometers use an ionizationchamber as a detector device. Such radiometers require the use of DC.amplifiers having a very high gain. Therefore, their stability over aperiod of time is poor, they are cumbersome, and have a high risk offailure due to intricate electronic circuitry. These shortcomings,together with their high cost, limit the devices to use as speciallaboratory apparatus where their extended dynamic measuring rangewithout saturation is desireable.

More currently, radiometers have been made using counting tubes asdetectors which operate on the well known Geiger-Muller principle. Suchdetectors will be called hereinafter GM-tubes." In a widely used type ofapparatus, the GM-tube is supplied with a stabilized DC. voltageslightly above the Geiger-Muller threshold voltage. Under the action of-y-radiation, it yields pulses of even amplitude or shots at a recurringfrequency which is an increasing function of the received amount ofradiation. After shaping of the pulses, an integrating circuittransforms this recurring frequency into a direct current, the strengthof which measures the radiation.

This type of radiometer is thus rather simple, inexpensive and has goodprecision and stability over a period of time. However, it has a majordrawback which limits its use. This is due to the complex variation ofthe counting rate" (number 'of shots per second) as a function ofradiation quantity,

wherein for small values and over a range of about 10 decibels (dB), theresponse is linear, then, for larger quantities and over another dB, agradual saturation makes the response almost logarithmic and eventuallyfull saturation occurs. When these three ranges are taken into account,it will be understood that it is difficult to provide an apparatushaving simple electronic circuitry and easy maintenance while also beingregarding the shape of the response curve.

able to serve the whole dynamic range of about 25 dB extending from thethreshold of detection to saturation.

Attempts to obviate these drawbacks of GM-tube type radiometers haveused several operating modes with switching between six ranges, eachcovering 10 dB. The interference of the saturation effect was avoided byeliminating use of a DC. voltage for measuring large quantities and bysubstituting voltage pulses having decreasing widths in the higherranges. Radiometers using such principles, however are very costly,cumbersome and have a heavy consumption of electricity which limitstheir self-sufiiciency.

In a common use of a GM-tube, it is possible to count the shots by anintegration operation external to the tube employing an ordinarymicroammeter to measure the mean current delivered under the action ofthe radiation. When the pulses that correspond to the shots are of evenamplitudes, it is obvious that the mean current will be proportional tothe counting rate. This type of radiometer, also has the same qualitiesand drawbacks of the shot-counter type, particularly Furtherimprovements have enlarged the dynamic range fo the logarithmic responseby adding the currents from two GM- tubes having different sensitivitiesand subjected to the same 'y-radiation. In principle, the less sensitivetube becomes effective to operate at an amount of radiation whichcorresponds to the saturation of the more sensitive one. This permitsmeasurements of about 30 dB in a logarithmic range which can be read outon one common dial, without any range switching. in addition, a lineargraduation can be added at the beginning of the scale for smallradiation values, which brings the entire range of the radiometer to 40dB. In practice, this very simple device has two drawbacks: it requiresvery stable tubes having like variations over a period of time; andthere is a risk of great error in the measurements when the lesssensitive tube fails.

Of course, three, four, or more GM-tubes can be used and the currentstherein added, but the indicated drawbacks rise when the number of tubesincreases.

SUMMARY OF THE lNVENTlON .The object of this invention is therefore toprovide a multiple GM-tube which will correspond to a set of severalindependent GM-tubes having different responses but with a singlefilling of gas, and common envelope to avoid the drawbacks of thesimplified radiometers using independent GM-tubes, such as the lack ofstability and reliability.

According to a first feature of a multiple GM-tube of the presentinvention, an anode in the form of a wire is common to several sensitivestructures or spaces. A first cylindrical cathode, having a small lengthand coaxial with the anode defines a first space, sensitive to thestrongest radiations; a second cylindrical cathode, having a length anda diameter greater than the first one and also coaxial with the anode,defines a second space sensitive to radiations of less strength, withthe sequence of such cathodes, coaxial to the anode, continuing up tothe last one which has the largest measurements and is sensitive to theweakest radiations to be measured.

According to another feature of the invention, the relative sizes of thecathodes are determined so that each of the sensitive spaces detectsamounts of radiation that vary within a predetermined range of dB, everyspace reaching its saturation for an amount which corresponds to thethreshold of logarithmic operation of the adjacent space of lesssensitivity so that the curves for each sensitive space, shows a partialcurrent as a function of the radiation in terms of decibels and arerepresented by parallel straight lines between the threshold oflogarithmic operation and saturation.

According to another feature of the invention, the adjacent sensitiveareas are separated by an insulating stack which prevents the ionizationin any one of theareas from being propagated toward the others so thatthe various indications of radiation in the several areas are keptindependent.

According to another feature of the invention, the anode wire includesseveral auxiliary elements or a protrusion or bend, one for each area,except the smallest least sensitive sec-' tion, so as to effectivelybring the wire somewhat closer, at particular portions of its length, tothe corresponding cathode, in order to equalize the threshold voltagesfor the Geiger- Muller operating conditions in the various spaces.

According to another feature of the invention, the cathodes areinsulated from one another and each provided with an output lead, sothat the current in each section can be measured separately before theyare summed up in a common wire. The common wire which is connected toall the cathodes then adds the mean currents to each section to providea measure of radiation on a single meter scale without range switching.

Further objects and features of the invention will appear in thefollowing specification referring to the accompanying drawings whichshown, by way of example, a particular embodiment of the presentinvention and illustrates the operation thereof.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I shows a double section GM-tubeaccording to the invention, designed for measuring Gamma radiationsbetween 50 mR/h and 500 R/h,

FIG. 2 is a diagram which shows the curves of the partial currentsdetected in each sensitive space as well as their sum as a function ofradiation measured in dB,

FIG. 3 is a schematic representation of a radiometer arrangement usingthe GM-tube according to the invention,

FIG. 4 shows a tube with three sensitive sections, and

FIG. 5 shows a tube with two sensitive sections and spiral shapedcathodes adapted to measure Beta radiation.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The double section GM-tubeshown in FIG. 1 is comprised of a glass envelo e 1 having an innerdiameter of the order of I5 mm. and a thickness of about 1 mm. One endhas a pinched base 2 including three air-tight output leads 3, 4 and 5,and the other end has a tip 6 which is sealed after exhaustion. Anextension of the central lead 4, preferably of ferrachrome, forms theanode wire 7 having a diameter of about 0.8 mm. Insulating beads 8, 9,and 11 of sintered glass are threaded on anode 7. Beads 8 and 9 areconnected by a cylinder 12 which forms the first cathode and defines thefirst sensitive space. Beads 9 and 10 are adjacent one another. Beads 10and l l are connected by a cylinder 13 of a substantially greater sizethan cylinder 12 and which forms the second cathode defining the secondsensitive space. Cylinders 12 and 13 are preferably both of ferrochromeor other suitable conducting material which will not absorb the gasfilling the envelope 1.

Anode 7 is provided with a small auxiliary element such as a cylinder orplate 14, which may also be of ferrochrome, designed to enhance theexchange of ions between anode 7 and cathode 13. The elements are of asize adapted to equalize the values of the operation points,particularly the threshold voltages in structure 7, 12 on one hand andstructure 7, 13 on the other hand, which define respectively the firstspace of relatively low sensitivity and the second space of highersensitivity.

The electrical connections between cathodes l2 and 13 and their outletleads 5 and 3 may also be made of ferrochrome wires 15 and 16respectively, which are welded electrically.

The whole of the inside structure is secured to the anode by an eyeletI7, preferably of ferrochrome, welded electrically. The circumference ofbead 11 rests on the tapered end of the glass envelope 1. Any othersuitable known supporting means, can also be used.

The whole of the structure is filled with a mixture of one or more raregases and halogen vapors, as used in conventional counting tubes. Themembers forming the structure including the sintered glass beads, fittogether in a manner which permits free circulation of the gases.

A particular example of a double section GM-tube is designed to measureradiation between 50 mR/h and 500 R/h on a single scale withoutswitching. The sizes of the cathodes are as follows: cathode l2, innerdiameter 5 mm, length4 mm; cathode l3, inner diameter 10 mm, length 60mm, with wall thicknesses of 0.2 mm and 0.25 mm, respectively.

The operation of the device is in accordance with the principles of aconventional GM counting tube when the supply voltage exceeds apredetermined value called the Geiger- Muller threshold voltage. It isknown that each ionizing event, resulting from a particle or a 'y-photonwhich passes through the gas, generates an electron. This causes acomplex ionization phenomenon and propagation of a discharge whichresults in a voltage pulse called a shot." The time for restoring thenormal conditions of the electric field near the anode wire, for thelatter to operate again under the action of another ionizing event, isof the order of a few tens of microseconds; when such a second eventoccurs during that dead time, it is not counted.

For small values of radiation, the counting rate N, i.e. the number ofshots per second, is proportional to the quantity so that a sensitivitys N/R/h can be defined. Following this linear region, which extends froma practical readable threshold through a dynamic range of about 10 dB ofradiation, the counting rate N increases more slowly because ofincreasing saturation from the effects of the dead times, and varies asa function of R/h in accordance with approximately a logarithmicresponse through a range of about I5 dB. Eventually, a total saturationoccurs.

The sensitivity s is approximately proportional to the effective surfaceof the GM-tube, i.e. the area of a rectangle obtained by theintersection of the cathode cylinder and a plane which comprises theaxis of the latter. When considering two conventional simple GM-tubesthat only differ as to their ef- I fective surface (the ratio betweenthe two surfaces being assumed to be r), their sensitivities will havethe same ratio r, and in the linear region, the two GM-tubes will havethe same counting rate N for radiation amounts that will also have thesame ratio r.

Experience shows that what has been stated above also occurs in theregion of logarithmic response and at saturation. In other words, whenresponse curves a and b of two GM-tubes are drawn in the same diagram,as shown in FIG. 2, with the counting rates N in ordinates anddeliveries of amount R/h, measured in decibels, in abscissae, it can bestated that curves a and b can be superimposed by a mere translationparallel to the axis of abscissae and amount to 10 log r.

This is also true when current I, which the GM-tube delivers under theaction of the radiation, is measured instead of counting the shots persecond. Since the pulses which correspond to the shots are of evenamplitude, the mean current I is proportional to the number of thesepulses in a unit of time. Therefore, the abscissae in FIG. 2 can begraduated in terms of current I as well as of counting rate N. When thetwo currents corresponding to the two simple GM-tubes are summed up,assuming that the extent of each of the regions has a quasilogarithmicresponse range of p dB represented by the straight portions of curves aand b in FIG. 2 which amount to 10 log r, it can be stated that theresulting curve c'in FIG. 2, drawn as a dotted line, is a straight linewith a range of 2p dB or about 30 dB. The ratio r between the effectiveareas of the two simple GM-tubes is thus of about 10" or 30.

Taking into account the measuring principle, it is essential that therelative positions of curves 4 and b in FIG. 2 remain constant,otherwise the resultant curve 0 can be mishaped. This may happenordinarily when the relative variations of sensitivity of the two simpleGM-tubes, which may change with conditions of use and aging occur inopposite directions. Regarding this point, the double GM-tube accordingto the present invention does not have this disadvantage since the twosensitive spaces bounded by cathodes l2 and 13, which are equivalent tothose of two separate GM-tubes, contain a common gaseous compound, aresubjected to the same supply voltage, have the same Geiger-Mullerthreshold voltage and have the same relative position with respect tothe radiation to be measured. When the conditions of operation such asthe supply voltage, change, it has been found experimentally that thesensitivities vary with the same relative value in both sec- 'tions.This appears on the diagram of FIG. 2 as a translation of the entirecurves a, b, c in a direction parallel to the axis of abscissae. lt canbe noted also that the lower portion of curve a, which corresponds to asmall amount of radiation, is translated in the same direction.Therefore, the overall response range is about 40 dB, including 10 dB ina linear graduation at the beginning of the scale and then up to 30 dBin a logarithmic graduation to saturation. This type of operationprovides a radiometer which is simple to use, maintain and calibrate.

These advantages will be further understood 'by referring to FIG. 3which schematically shows a radiometer using the double GM-tubeincluding a glass envelope 1, anode 7 with its outlet 4, cathode 12 thatsurrounds the smaller sensitive space with its outlet 5, and cathode l3surrounds the larger sensitive space, with its outlet 3. The stabilizedhigh positive voltage, of about 400 volts, is applied to anode 7 by asupply device 18, of a suitable known type which may include the usuallow voltage source, a step-up transformer for direct voltage and avoltage stabilizer. A resistor 19 is connected to outlet 5 of cathode 12and a resistor 20 to outlet 3, of cathode 13 the values of 19 and 20being of approximately 2 megohms. Resistors 19 and 20 have a commonpoint to add the cathode currents and are. connected back to a referenceor ground potential through a microammeter shunted by an adjustableresistor 22. The

microammeter scale is graduated linearly through a range of about dB andthen logarithmically through a range of 30 dB, and is marked inRoentgens per hour.

When a 'y-radiometer having the present double GM-tube is calibrated,the tube is subjected to a standard high radiation but of a lower valuethan would saturate the less sensitive section. The resistor 22 is thenadjusted so that the pointer of microammeter 21 is set at a mark whichcorresponds to the standard radiation. This operation corresponds to atranslation of the whole of the curves of FIG. 2, as has been explainedabove. Thus the present GM-tube can be readily adapted, by an adjustmentof resistor 22 in presence of a standard radiation, to the user'sradiometer, with a source of stabilized voltage slightly different fromthe original calibration source. This feature also facilitates theperiodic calibrations of the radiometer. A most significant preciousadvantage of the double GM-tube is in the fact that there is no risk oferrors in the measurement of radiation, as can occur with the used twoseparate tubes when one of them is faulty, since in this case anyfailure affects both sensitive spaces at the same time.

A GM-tube having three sensitive spaces designed according to thepresent invention should have a range of about 55 dB. Such a tube isshown in FIG. 4. This includes a glass envelope 1, having a base 2including four air-tight output leads 3, 4, 5 and 20 and, at its otherend, a tip 6 which is closed after use in exhausting. An extension ofthe central lead 4 in ferrochrome forms the anode wire 7. Insulatingbeads 8, 9, l0, l1, l7 and 19 of sintered glass are threaded on anode 7.Beads 17 and 19 are connected by a cylinder 18 which forms the firstcathode and defines the first sensitive space. Beads 17 and 8 areadjacent one another. Beads 8 and 9 are connected by a cylinder 12 of asubstantially greater size than cylinder 18 and which forms the secondcathode and defines the second sensitive space. Beads 9 and 10 areadjacent one another. Beads 10 and 11 are connected by a cylinder 13 ofa substantially greater size than cylinder 12 and which forms the thirdcathode and defines the third sensitive space. Cylinders 12, 13 and 18are connected to leads of ferrochrome or any other conducting materialwhich will not absorb the filling gas in envelope l. Anode 7 is providedwith a small cylinder or plate 22, also of ferrochrome, designed toenhance exchange of ions between anode 7 and cathode 14, with a bend orprotrusion 23 similarly designed for enhancing exchange of ions betweenanode 7 and cathode 13. Elements 22 and 23 are designed as to equalizethe values of the operating points, and namely the threshold voltages inthe structures 7-18, 7-12 and 7-13 which define respectively the firstspace of smaller sensitivity, the second space of medium sensitivity andthe third space of greatest sensitivity.

This GM-type having three sensitive space, is designed to reach a rangeof 55 dB, or mother words can measure Gamma radiation between 3 mR/h and1,000 Mb. Examples of the cathode dimensions are as follows:

Cathode l8 diameter, 2 mm; length, 2 mm;

Cathode l2 diameter, 7 mm; length, 10 mm;

Cathode l3 diameter, 15 mm; length mm.

Other forms of multiple GM-tubes are adapted to measure the strength ofcomplex nuclear radiations and may use spiralshaped cathodes instead ofthe full-wall cathodes previously described or can use a gaseous fillingwhich makes them responsive to neutron radiations. One such tube havingtwo sensitive spaces and designed for measuring Beta radiations is shownin FIG. 5. The cathodes l2 and 13' are spiral or helical shaped in orderto allow a large amount of Beta-radiations which have passed through theenvelope 1, to cross freely. Envelope 1 is made thinner in the part 24which is facing cathodes l2 and 13'. Except for elements 12', 13 and 24,the other components and dimensions of the GM-tube shown in FIG. 5 arelike the ones of the GM-tube shown in FIG. 1.

Although the principles of the present invention have been cathodes eachhaving different areas and sensitivities disposed in spaced relationalong the coaxially surrounding said anode, said cathodes being disposedwithin and spaced from said en velope.

2. The radiation counting tube of claim 1 wherein said anode is alongitudinal wire common to said coaxialcathodes, and separate outputleads are connected to each said cathode and said anode.

3. The radiation counting tube of claim 2 wherein said cathodes havecylindrical shapes of different lengths and diameters forming aplurality of different radiation sensitive sections about said anode,the larger lengths and diameters providing greater sensitivities.

4. The radiation counting tube of claim 3 wherein said cylindricalcathodes have helical shapes with spaces for the passage of ions.

5. The radiation tube of claim 3 wherein each of said cathodes aresupported on glass discs disposed transversely of the axis of saidenvelope and at each end of the associated one of said cathodes,adjacent ones of said discs of adjacent ones of said cathodes being incontact with each other, each of said discs having an axial hole tosupport said anode and said discs associated with each of said cathodesconfining the resulting ionization of their associated one of saidplurality of radiation sensitive sections to prevent propagation of saidresulting ionization into others of said plurality of radiationsensitive sections.

6. The radiation counting tube of claim 3 wherein said tube is of theGeiger-Muller type having a threshold voltage for operation andincluding means within the larger said cathodes positioned between saidanode and said larger of said cathodes to equalize the thresholdvoltages for each section.

7. The radiation counting tube of claim 3 including voltage supply meansconnected between said anode and cathodes, means connecting saidcathodes together, and a common current measuring means connected inseries with said voltage supply means and said means connecting saidcathodes to provide a single measuring scale without range switching.

10. The radiation counting tube of claim 7 wherein each cathode issensitive to a predetermined range of radiation and the ranges are addedtogether in said common meuuring means.

1. A radiation counting tube comprising a hermetically sealed envelope,a gas disposed in said envelope, an anode disposed coaxially of saidenvelope, and a plurality of cathodes each having different areas andsensitivities disposed in spaced relation along the coaxiallysurrounding said anode, said cathodes being disposed within and spacedfrom said envelope.
 2. The radiation counting tube of claim 1 whereinsaid anode is a longitudinal wire common to said coaxial cathodes, andseparate output leads are connected to each said cathodes and saidanode.
 3. The radiation counting tube of claim 2 wherein said cathodeshave cylindrical shapes of different lengths and diameters forming aplurality of different radiation sensitive sections about said anode,the larger lengths and diameters providing greater sensitivities.
 4. Theradiation counting tube of claim 3 wherein said cylindrical cathodeshave helical shapes with spaces for the passage of ions.
 5. Theradiation tube of claim 3 wherein each of said cathodes are supported onglass discs disposed transversely of the axis of said envelope and ateach end of the associated one of said cathodes, adjacent ones of saiddiscs of adjacent ones of said cathodes being in contact with eachother, each of said discs having an axial hole to support said anode andsaid discs associated with each of said cathodes confining the resultingionization of their associated one of said plurality of radiationsensitive sections to prevent propagation of said resulting ionizationinto others of said plurality of radiation sensitive sections.
 6. Theradiation counting tube of claim 3 wherein said tube is of theGeiger-Muller type having a threshold voltage for operation andincluding means within the larger said cathodes positioned between saidanode and said larger of said cathodes to equalize the thresholdvoltages for each section.
 7. The radiation counting tube of claim 3including voltage supply means connected between said anode andcathodes, means connecting said cathodes together, and a common currentmeasuring means connected in series with said voltage supply means andsaid means connecting said cathodes to provide a single measuring scalewithout range switching.
 8. The radiation counting tube of claim 6wherein said means to equalize said threshold voltages includes acoaxial ring around said anode.
 9. The radiation counting tube of claim6 wherein said means to equalize said threshold voltage within thelargest diameter cathode includes a bend in said anode.
 10. Theradiation counting tube of claim 7 wherein each cathode is sensitive toa predetermined range of radiation and the ranges are added together insaid common measuring means.